I. Field
The present invention relates to wireless voice and data communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for adaptively controlling power levels of data transmissions.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term “cellular” systems encompasses both cellular and personal communications services (PCS) frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems for data, etc. are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and fully incorporated herein by reference. In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same radio frequency (RF) band. For example, each remote subscriber unit (e.g., a cellular telephone, personal digital assistant (PDA), laptop connected to a cellular telephone, hands-free car kit, etc.) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with remote units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum. Transmitting signals over the same RF spectrum provides various benefits including, e.g., an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a remote unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station. An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In conventional cellular telephone systems, a public switched telephone network (PSTN) (typically a telephone company) and a mobile switching center (MSC) communicate with one or more base station controllers (BSCs) over standardized E1 and/or T1 telephone lines (hereinafter referred to as E1/T1 lines). The BSCs communicate with base station transceiver subsystems (BTSs) (also referred to as either base stations or cell sites), and with each other, over a backhaul comprising E1/T1 lines. The BTSs communicate with remote units via RF signals sent over the air.
To provide increased capacity, the International Telecommunications Union recently requested the submission of proposed methods for providing high-rate data and high-quality speech services over wireless communication channels. The submissions describe so-called “third generation,” or “3G,” systems. An exemplary proposal, the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), was issued by the TIA. The standard for cdma2000 is given in draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways.
The cdma2000 system uses a pilot channel and multiple traffic channels to carry voice and data services to subscribers. In order to optimize system performance on the reverse link between remote station and base station, pilot channel energies and traffic channel energies are balanced. Each channel is first spread with Walsh codes, which provides channelization and resistance to phase errors. A relative Walsh channel gain,
      F    =                  P        traffic                    P        pilot              ,is then added to the traffic channels in order to achieve a given Quality of Service (QoS). The optimal value for the Walsh channel gain is
            F      opt        =                            R          ⁢                                          ⁢          γ                          2          ⁢                                          ⁢                      B            ⁡                          (                              1                +                                  r                  ⁢                                                                          ⁢                  γ                                            )                                            ,where R is the data rate, B is the channel estimator bandwidth and γ is the Signal-to-Noise Ratio (SNR) needed for a decoder to achieve a desired Frame Error Rate (FER) at data rate R. (It is implicit that coding and antenna diversity are also used to achieve the desired FER.) This gain gives the optimal tradeoff between spending more energy on the pilot channel for good channel estimation and reducing pilot overhead. In practice, a pilot power level high enough for path searching must be selected, making F lower than Fopt. In addition, for some traffic channels in the cdma2000 system, power control may be performed based on information derived from the pilot channel. In some instances when the transmission rates may be unknown to the base station, the pilot power must be kept constant over the transmission rates, since the base station will perform power control based solely upon information derived from the pilot channel.
The data rate, interleaver length, and coding type determine the power ratio discussed above, regardless of the Walsh channel used. For a given rate r bps, the signal to noise ratio per bit (Eb/No=β in dB) required to achieve the given QoS includes the total transmitted power, which includes the pilot. For a system with chip rate c cps, and a given pilot to traffic ratio=ρ dB, the energy per chip divided by the interference spectral density (Ec/Io=σ in dB) can be found as:
  σ  =            10      ⁢                          ⁢                        log          10                ⁡                  (                                    10                                                (                                      β                    +                    ρ                                    )                                10                                                    1              +                              10                                  ρ                  10                                                              )                      -          10      ⁢                          ⁢                                    log            10                    ⁡                      (                          c              r                        )                          .            
It can be observed that the total amount of energy required to achieve a given QoS is dependent upon the velocity of the remote station. A method of estimating the power required to achieve a given QoS is described in U.S. application Ser. No. 09/519,004, entitled “VELOCITY ESTIMATION BASED GAIN TABLES,” filed on Mar. 3, 2000, assigned to the assignee of the present invention, and fully incorporated herein by reference. In the absence of velocity estimation, there is no way to estimate the amount of power required to achieve a given QoS. As an example of this phenomena, FIG. 1 displays the relationship between the Pilot/Traffic Power Ratio and Eb/No for remote stations at different speeds. In this example, the cdma2000 reverse link is operated with two receive antennas (i.e. two paths) for carrier frequency fc=2 GHz, chip rate=1.2288 MHz, data rate r=9600 bps, frame length 20 ms and 800-Hz power control by ±1 dB with 4% bit error rate (BER) on the power control bits and a delay of 1.25 ms. As shown by FIG. 1, a change in the speed of the vehicle causes the Eb/No required to achieve 1% FER to vary widely. At a stationary position, represented by line 100, the Eb/No levels, which are required to achieve a 1% FER, range between 2 dB and 2.5 dB. However, for a moving vehicle, as represented by lines 101, 102 and 103, the Eb/No required to achieve the same 1% FER range between 3 and 3.5 dB. Hence, for a 9600 bps reverse link, the Eb/No required to achieve 1% FER varies widely, which leads to a sub-optimal system. There is a present need for an improvement that corrects the inefficiencies caused by the motion of a remote station.